1. Field of the Invention
This invention relates to photovoltaic detectors and more specifically to a photovoltaic detector with integrated dark current offset correction.
2. Description of the Related Art
Photodetector arrays which may include hundreds or thousands of photovoltaic detector cells are used to detect intensity patterns in the infrared, ultraviolet and visible spectra in applications such as weather satellites, earth remote sensing, and industrial processes. A photovoltaic detector cell includes a flat photovoltaic wafer made from n-type or p-type crystalline semiconductor material that absorbs photons over a desired spectrum. The wafer is formed on a substrate that is transparent to photons in the desired spectrum. A thin surface layer of the opposite conductivity type is formed on the wafer so that the interface between the surface layer and the main or bulk region of the wafer defines a semiconductor p-n junction. A potential is applied across the cell such that the cell operates in reverse bias mode.
Illumination of the transparent substrate with photons having wavelengths in the desired spectrum creates electron-hole pairs in the cell that diffuse to and are collected at the p-n junction. This mechanism generates a photocurrent that is proportional to the intensity of the incident photons. Thermal energy also generates electron-hole pairs in both the bulk and depletion regions that diffuse to and are collected at the p-n junction. This mechanism generates a dark or leakage current that is independent of the illumination intensity and which adds to the photocurrent. The dark current increases as the band gap energy of the cell is narrowed. For example, in infrared detectors the dark current may be 5 to 10 times greater than the photocurrent. The dark current is also highly sensitive to changes in temperature and increases exponentially as the temperature increases. In high temperature industrial applications the dark current may effectively swamp out the photocurrent. In general the dark current is insensitive to changes in the potential across the cell. However, at low enough temperatures the electron hold pairs in the depletion region tend to dominate such that the dark current does change with potential.
As a result, the photovoltaic detector cell generates a detection current having a signal component equal to the photocurrent and an offset equal to the dark current. In a typical IR focal plane array, a readout capacitor integrates the detector current for a preset integration period at which point the capacitor is discharged and reset. An A/D converter, suitably 14 bits, digitizes the voltage signal from the integration capacitor and passes it to a calibration circuit. The calibration circuit computes the offset and the slope of the cell's current v. photon intensity response curve and normalizes the output to a zero offset and reference slope so that each cell in the array has the same response. The fabrication of a HgCdTe infrared detector of the type described above is described in Reine et al. Semiconductors and Semimetals: Volume 18 Mercury Cadmium Telluride, Academic Press, Ch 6--Photovoltaic Infrared Detectors, pp. 246-256, 1981.
The dark current offset reduces the cell's sensitivity to changes in photon intensity, which reduces the resolution of the digitized voltage signal. First, the signal-to-noise ration (SNR) of the detector current is proportional to the square root of the integration period for charging the capacitor. The dark current reduces the integration period, and thus reduces the SNR. Second, approximately 6 bits of the A/D converter are used to digitize the offset. Those bits are effectively wasted, thus reducing the resolution of the signal component from 14 bits to 8 bits.
Due to the strong variation in dark current with temperature, many systems have strict requirements on the maximum spatial temperature variation across the focal plane array as well on the temperature stability itself. This is true even when the dark current is less than the photocurrent. To maintain the accuracy of the calibration circuitry, the spatial variation is often specified as less than 0.1K variation across the array, and the stability is specified as less than 0.1K change over 30 seconds. In addition, some systems have strict requirements on the time required to cooldown and stabilize the array to the point it can operate accurately. These requirements necessitate expensive packaging, cooling, and calibration techniques.